As I explained last week, raised beds have many advantages, but growing success depends on the soil and compost you use.
If you buy compost for containers, you'll know bags don't go far and raised beds need much more compost.
Even the smallest bed - 15cm deep and 1sq m - needs 300 litres, or five large bags, and a more standard 30cm-high bed requires double that. With each bag costing between £5 and £7, you'd end up with very pricey lettuce and strawberries. This growing media only has enough nutrient to last six to eight weeks and you'll almost certainly have to add liquid feeds too.
You also need to be careful when choosing compost at a garden centre. Compost from old bags may have lost much of its goodness, so only buy ones that look new. Avoid heavy bags stored outside, where they absorb rainwater and start to degrade. Ideally, check the texture of the material before buying. Dense compost easily becomes waterlogged and material with lots of bark or woodchip may be overly free-draining and impede root growth.
The highest price doesn't guarantee the best product, and manufacturers periodically change their formulation - what's good one year can disappoint the next. In 2014, Which? tested 26 products that are available for 2015 and will have the same formulation in 2016. Which? assessed germination rates and how young plants fared. Top marks were awarded to Verve multi-purpose peat-free compost, available from B&Q. Grow-Sure Seed and Cutting Compost received the dunce's cap, with Homebase John Innes No1 seedling compost and Waitrose and Alan Titchmarsh peat-free compost in close pursuit. New Horizons multi-purpose compost wasn't tested this time but has scored well in the past.
To reduce costs, you could mix in soil conditioners such as municipal green waste. Fifty-litre bags usually cost around £2.50. But you would need to increase nutrient supplements. One possibility, if you start now, is to layer the conditioner with raw kitchen scraps. See Kitchen Waste on my website and use the same method as for Composting in a Bag.
Assessing nutrient levels of any compost is difficult, but real soil and homemade compost always win. If possible, add this to any raised bed. Wherever you place a bed, break up the soil underneath to improve drainage and allow soil fauna to move up into the bed. Supplement your compost with topsoil or mole hills from the garden. And, whenever possible, dig in or top dress with homemade compost.
Builder's merchants also supply topsoil, but examine any before buying. Check structure, avoiding stony or clay batches, and take a pH test, making sure it's neutral - pH7 - or no lower than 6.5. You can't tell whether the soil contains pathogens or toxins that could harm your plants.
Converting an open bed to one or more raised beds is easier. Mark out the sides of the beds and the paths between them, then scoop out and pile up the soil from the paths. Once you've built the beds, add the path soil to them. You should end up with perfectly deep soil. Do this ASAP to let the ground settle in time for spring. When planting, add compost or pelleted chicken manure. If you're using farmyard manure, make sure it's very well-rotted.
With closer planting distances and constant use, all in a restricted space, a raised bed needs regular feeding, preferably with generous applications of homemade compost.
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